1. Field of the Invention
The present invention relates to a method of heat treating steel and more particularly, to an improved method of heat treating steel for use in the transmission gear of a vehicle. The method includes the steps of carburizing the steel, quenching the steel, reheating the steel using ammonia gas, and fusing the steel, whereby the resultant steel possesses excellent anti-abrasion properties and good contact fatigue strength and bending fatigue strength properties.
2. Description of Related Art
Various types of methods of heat treatment for steel for use in a transmission gear of a vehicle are known in the art. Generally, the transmission of a vehicle transfers the driving force from the engine to the wheels in various driving states such as vehicle load, the condition of the road and at a desired speed, etc. The transmission includes gears which have various speed reducing ratios for changing the revolution speed and driving torque transmitted to the driving wheels. The gears, including the reverse gear, require excellent anti-abrasion and anti-cracking properties because the gears are engaging each other on almost a continuous basis when driving.
As shown in FIG. 1, the conventional carburizing methods for heat treating steel comprises the steps of (a) carburizing the steel using a carburizing agent at a temperature 900-950.degree. C., (b) quenching the steel by cooling it to a temperature of 830-850.degree. C. and (c) fusing the steel at a temperature of 150-230.degree. C. to provide the steel and transmission gears made therefrom with good anti-abrasion and good contact fatigue strength properties.
In the carburizing step (a) of such a conventional method, the surface of the treated transmission gear becomes a high carbon steel and the internal portion of the steel becomes a low carbon steel. Thus, the gears made from this steel possess only anti-abrasion and contact fatigue strength properties. The quenching step (b) achieves the hardness of the surface of the transmission gears by changing the steel from an austenite state, which melts carbon and other elements in .lambda. steel of the transmission gears, to the martensite state. The fusing step (c) increases the unstable structure-condensing ability, and the expanding and contracting ability.
However, the transmission gears produced by such a conventional method (FIG. 1) suffers from a number of problems. For example, since the treated transmission gears contain a granular carbonate, a large sized crystal grain forms, and austenite is distributed throughout the entire steel in a large amount. Due to these occurrences, transmission gears produced by such a conventional method do not have satisfactory anti-abrasion and contact fatigue strength properties.
In order to solve the above problems, another conventional method of heat treating steel comprises: (a) carburizing the steel by using a carburizing agent at a temperature of 900-950.degree. C. (b) quenching the steel in the presence of ammonia gas to a temperature of 830-850.degree. C., and (c) fusing the steel at a temperature of 150-230.degree. C. (FIG. 2 ). In step (b), the contact fatigue strength can only be improved by retaining elemental nitrogen through the addition of ammonia gas. Thus, the retained austenite maintains its balance.
However, transmission gears produced by such a conventional method (FIG. 2) suffer from a number of problems. For example it is difficult to obtain excellent anti-abrasion and contact fatigue strength properties, and it is difficult to control the furnace due to the presence of ammonia gas, whereby the product does not have good quality.